1. Field of the Invention
The present invention relates to a laser diode module used as a light source for exciting an optical fiber amplifier.
2. Description of the Background Art
FIG. 13 schematically shows a structure of a conventional laser diode module disclosed in U.S. Pat. No. 5,724,377. The laser diode module includes a laser diode 1 and an optical fiber 12 located on an optical path of light emitted from laser diode 1. Laser diode 1 emits light with a wavelength of 982 nm when there is no optical feedback. Optical fiber 12 has a microlens part 13 at its tip where light is incident as well as a part where a fiber grating 4 is formed that reflects light with a center wavelength of 980 nm. In general, the fiber grating is formed to cause refractive index to periodically change in the optical fiber. Because of the periodic change of the refractive index, the fiber grating has characteristics that it only reflects a constant amount of light having a specific wavelength according to the rate of change in refractive index. The fiber grating can be manufactured by exposing an optical fiber to ultraviolet radiation.
A laser beam emitted from laser diode 1 enters microlens part 13 to be converted from diffused light into parallel light and propagates as the parallel light through the inside of optical fiber 12. The laser beam passing through optical fiber 12 to reach fiber grating 4 is reflected from fiber grating 4 to be returned to laser diode 1. In a composite resonator thus structured, the wavelength of laser diode 1 is not 982 nm as indicated above but 980 nm which is the center wavelength of fiber grating 4, and the wavelength of laser diode 1 is locked or fixed on this wavelength. In other words, regardless of forward current applied to laser diode 1 and ambient temperature of the laser diode, a stable optical output with a wavelength of 980 nm can be achieved.
FIG. 14 shows a relation between optical output Pf from an end of the fiber and forward current If of the laser diode to compare the laser diode module having fiber grating 4 within optical fiber 12 thereby returning the laser beam to laser diode 1 as described above with a laser diode module with no fiber grating. The laser diode module without fiber grating has characteristics indicated by the dotted line while the laser diode having fiber grating 4 has characteristics indicated by the solid line. It is apparent from FIG. 14 that, regarding the laser diode module having fiber grating 4, the maximum optical output which is possible without occurrence of kink (hereinafter referred to xe2x80x9cmaximum kink-free optical output powerxe2x80x9d) remarkably decreases. A resultant problem is that the available optical output of the laser diode module having the fiber grating is nearly a half of the optical output which is possible by the laser diode module with no fiber grating. xe2x80x9cKinkxe2x80x9d refers to a phenomenon that occurrence of beam steering eliminates linearity between optical output within the optical fiber and forward current applied to the laser diode, which is detailed later.
One object of the present invention is to solve the problem above by providing a laser diode module enabling a sufficiently great maximum kink-free optical output power even when a wavelength to be used is locked on a desired value by means of a fiber grating.
According to one aspect of the invention, for the purpose of achieving the object above, a laser diode module of the present invention includes a laser diode, a lens located on an optical path of light emitted from the laser diode, and an optical fiber located to receive the emitted light concentrated by the lens. The optical fiber includes a fiber grating having a reflection center wavelength greater than cutoff wavelength of the laser diode and having a half-width of reflection spectrum ranging from twice the spacing between longitudinal modes of the laser diode to 10 nm.
When this structure is employed, the reflection center wavelength of the fiber grating is greater than the cutoff wavelength. Accordingly, fundamental transverse mode dominates high-order transverse mode thereby suppresses high-order transverse mode oscillation. Further, the fiber grating has its half-width of reflection spectrum that is at least twice the spacing of longitudinal modes and thus the fiber grating has a reflection band containing a plurality of longitudinal modes of the laser diode. Oscillation of a plurality of longitudinal modes stabilizes optical output. In addition, the fiber grating has a half-width of reflection spectrum that is 10 nm or less, the wavelength can be controlled to a precision of 10 nm or less.
According to another aspect of the invention, a laser diode module includes a laser diode, a lens located on an optical path of light emitted from the laser diode, and an optical fiber located to receive the emitted light concentrated by the lens. The optical fiber includes a fiber grating having a reflection center wavelength greater by at least 2 nm than gain peak wavelength of the laser diode and having a half-width of reflection spectrum ranging from twice the spacing between longitudinal modes of the laser diode to 10 nm.
By employing this structure, the maximum kink-free optical output power can efficiently be increased since the maximum kink-free optical output power sharply increases as seen from the experimental result shown in FIG. 5 when the difference between the reflection center wavelength and the gain peak wavelength (oscillation center wavelength of the laser diode itself) exceeds 2 nm.
Preferably, when a fundamental transverse mode parallel to a crystal growth layer of the laser diode matches a fundamental transverse mode of the optical fiber at a magnification called fundamental transverse mode matching magnification, coupling magnification of the laser diode and the optical fiber has a value between the fundamental transverse mode matching magnification and a magnification producing a maximum coupling efficiency. As seen from the experimental result shown in FIG. 11, the maximum kink-free optical output power can be increased to a greatest value by employment of this structure.
Preferably, the laser diode is of ridge waveguide type and produces oscillation at 980 nm. This structure can be used to apply the laser diode to use as an optical source for excitation of an optical fiber amplifier.
Preferably, the lens is a cylindrical lens provided on a light-incident end of the optical fiber. When this structure is employed, the magnification is 1 with respect to the direction perpendicular to the direction where lens effects are exhibited. High-order mode oscillation can accordingly be suppressed and thus the maximum kink-free optical output power increases.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.